1. Field of the Invention
The present invention relates to an X-ray fluoroscopic inspection apparatus. More particularly, the invention relates to an X-ray fluoroscopic apparatus which is applicable to an apparatus for displaying a fluoroscopic image of an object to be fluoroscoped on a monitor screen or radiographic fluoroscopic image, and also to an X-ray CT apparatus for constructing a CT image from the fluoroscopic image.
2. Description of the Related Art
In the X-ray fluoroscopic apparatus, such as an industrial X-ray fluoroscopic inspection apparatus, an X-ray camera containing an image intensifier, a CCD camera and the like is disposed in opposition to an X-ray source. An object (referred to as a sample), such as a sample, is located between the CCD camera and the X-ray source. The X-ray camera radiographs an X-ray fluoroscopic image of the sample.
Normally, a sample table is disposed between the X-ray source and the X-ray camera. The sample table positions the sample so that a desired viewpoint on the sample is located within a visual field of the X-ray camera. The sample table is movable in two axes (x-axis and y-axis), perpendicular to each other, on a plane along a surface of the table. The sample table is rotatable about a z-axis perpendicular to the x- and y-axis, viz., extending in an optical axis of the X-rays emitted from the X-ray source.
This kind of X-ray fluoroscopic apparatus is equipped with a mechanism which moves the X-ray camera and the sample table in an optical axis (z-axis) direction of the X-rays emitted from the X-ray source so as to vary a magnification factor of the fluoroscopic image as desired. Some type of the X-ray fluoroscopic apparatus is further equipped with a mechanism which tilts the X-ray camera in a predetermined direction with respect to the optical axis of the X-ray source so as to enable the camera to fluoroscope the object at an inclination angle.
The X-ray fluoroscopic apparatus with the tilting mechanism involves the following problem. An operator places a sample on the sample table, and positions the sample table so as to have a desired viewpoint on the sample in a state that the X-ray camera is positioned on the optical axis of the X-ray, for example. Then, the operator tilts the X-ray camera to fluoroscope the sample at the inclination angle. At this time, the viewpoint on the sample moves within the visual field or moves out of the visual field, disadvantageously. In this case, even if the sample table is moved in the x-axis or the y-axis direction so that the viewpoint falls within the visual field of the X-ray camera, a distance (SOD) between the X-ray source and the sample at the position of the table before the camera is tilted will change from the original distance. Therefore, even if a distance (SID) between the X-ray source and the X-ray detector is fixed, a fluoroscopic magnification factor represented by SID/SOD changes from its value before the camera is tilted.
In a case where a rotating mechanism for rotating the sample table is put on the mechanism for moving the table in the x-axis and the y-axis directions, the following problem arises. When the sample is put on the sample table and the viewpoint on the sample is positioned so as to be located at the center of a visual field of the camera, and the table are rotated, the viewpoint is sometimes greatly shifted from the center of the visual field of the X-ray camera since the rotary shaft of the sample table has been moved in the x-axis and the y-axis directions. In such a case, when the sample table is rotated, the viewpoint moves out of the visual field while describing an arc.
There is known a technique to solve that problem (see JP-A-2001-249086 (pp 3 to 12)). In the technique, the correction amounts of the movement of the sample table in the x-axis and the y-axis directions are successively computed so as to keep the original position of the viewpoint within the visual field when the X-ray camera is tilted or the sample table is rotated. The moving mechanism for the x- and y-axis direction movements is driven by using the computing results.
Where this technique is used, the viewpoint always stays within the visual field of the X-ray camera even if the X-ray camera is tilted or the sample table is positioned and rotated. In particular, an efficiency of the work of fluoroscoping the sample at high magnification factor is considerably improved.
The above-mentioned related-art tracking technique has the following disadvantage. A set of x and y coordinates of the position on the table at which the operator first sets the sample will change in x and y directions by quantities of the movement-amount correction after the camera is titled or the table is rotated. This makes it difficult for the operator to use values of the x and y coordinates as the reference for fluoroscopic visual field position on the sample.